Fabric of commingled fiberglass and polytetrafluoroethylene and method of producing same

ABSTRACT

An improved composite fiber of fiberglass and polytetrafluoroethylene (PTFE) is provided. The composite fiber comprises a coherent strand of commingled filaments of fiberglass and PTFE which is far more resistant to flex, abrasion, and chemical attack than previous fiberglass fibers. Preferably filaments of expanded PTFE tow yarn and filaments of PTFE are combined through a process of air-jet texturing. The fibers of the present invention have a wide range of possible uses, including being formed into a fabric and employed as filter media.

RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 08/307,539 filed Sep. 16, 1994, now allowed U.S. Pat. No. 5,549,960which is a division of U.S. patent application Ser. No. 08/033,678 filedMar. 16, 1993, which is abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improved composite fibers and fabricsand methods for producing them. More particularly, the present inventionrelates to an improved composite textile of fiberglass andpolytetrafluoroethylene (PTFE).

2. Description of Related Art

Fiberglass textiles are widely employed today in numerous applications.This is due to the many favorable properties of fiberglass, includingrelatively low cost, excellent mechanical strength, dimensionalstability, good insulative properties, and excellent resistance totemperature and humidity. Among the popular present uses of fiberglassare as an architectural fabric, in the electronics industry (e.g. as aprinted circuit board), as a filtration media, and as a structuralcomposite.

Due mostly to its relatively low cost, fiberglass presently dominatesthe high temperature filtration market (typically defined as thosefilters operating within 300° to 500° F.). In this regard, fiberglassoften costs 33 to 95% less than some competitive synthetic textilefibers (e.g. polytetraethylene, aramids, polyphenylene sulfide,polyimides and copolyimides).

Although fiberglass performs adequately in these applications, itsuffers from numerous deficiencies. Perhaps the greatest problem withfiberglass is that it is quite fragile-being easily damaged when flexedor abraded. As a result, fiberglass fabric is incapable of performing inmany applications. For instance, where a filter must be vigorouslyflexed to free filtrate and clean the filter media (e.g. in a"shaker"-type bag house filter or a pulse-jet bag house filter), afiberglass fabric will quickly fail. Even under less demandingregenerative procedures (e.g. reverse air cleaned bags), where far lessbag flex is encountered, fiberglass has relatively limited operationallife.

Another deficiency of fiberglass is that it is subject to certainchemical attack. Chemicals attack the glass filaments in the textileprimarily when the process gas phase goes through a dewpoint excursion.Chemical attack can also occur in the gaseous phase or when solidparticulate contacts the fiberglass.

In an effort to address some of the problems of chemical attack, anumber of finishes and protective coatings have been developed. Examplesof such treatments include constituents of silicone oils, graphite andPTFE dispersion coatings. Although such treatments have provenrelatively effective at protecting fiberglass from chemical attack, theydo not improve the problem of flex failure.

Another common problem with fiberglass is that it is difficult to handleand work. By way of example, if the fiberglass is not carefully handledduring a weaving process, the low abrasion resistance of the fiberglassoften leads to friction damage to the glass fibrils. Likewise, abrasioncaused by weaving equipment can result in surface imperfections (e.g.glass pills) which can cause problems in later processing. Further, evena single fold in the glass material during processing can lead tofatigue and later failure.

In light of these many problems, synthetic fibers are the fibers ofchoice where long filter life is needed and/or extreme operativeconditions are expected. Unfortunately, the costs of synthetic fibersrestrict their accepted uses. Additionally, certain synthetic fibers,while having many exceptional characteristics, are also limited incertain respects. For example, PTFE tends to have stability (creep)problems in reverse air bag houses, with tension on the bag elongatingit to a point at which proper cleaning and flexure does not occur.Copolyimides and aramids hydrolyze in the presence of moisture and hightemperatures. Polyphenylene sulfide oxidizes and embrittles when exposedto oxygen and high temperature.

Accordingly, it is a primary purpose of the present invention to providean improved fiber composite which retains many of the desirableproperties of fiberglass, but which is far more resilient to flex,abrasion, and chemical attack.

It is a further purpose of the present invention to provide a fiberglasscomposite material which can be thoroughly handled and worked withoutfear of compromising its structural integrity.

It is another purpose of the present invention to provide a relativelylow-cost composite material which has many of the desired properties ofexisting synthetic materials, such as long product life and resistanceto chemical attack.

These and other purposes of the present invention will become evidentfrom review of the following specification.

SUMMARY OF THE INVENTION

The present invention is an improved composite fiber ofpolytetrafluoroethylene (PTFE) and fiberglass and a method for producingit. By commingling filaments of fiberglass within filaments of PTFE, acomposite fiber is created which has many of the benefits of fiberglassfibers, but with far greater flexibility and resistance to chemicals andabrasion.

In the preferred embodiment of the present invention, the compositefiber is created by combining an expanded PTFE tow yarn and a fiberglassyarn in an air-jet texturing apparatus. By so combining, the filamentsof expanded PTFE become intertwined around the filaments of fiberglassso as to form an single strand of strong, flexible composite material.The material can be readily formed into a fabric with a wide range ofpossible uses, including as a filter media, as an architectural fabric,as a structural fabric (such as when combined with an epoxy resin), etc.

The fiber and fabric of the present invention has numerous advantagesover previously available fiberglass materials. For example, the fiberis sufficiently resistant to flex and abrasion that it can be easilyhandled and worked without compromising its structural integrity,Additionally, the combination of fiberglass and expanded PTFE imparts anumber of improved properties to the composite which have previouslyeluded some synthetic fabrics, including greater temperature andmoisture resistance, greater stability and resistance to creep, andreduced cost.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is an enlarged, partially schematic cross-sectionalrepresentation of one embodiment of a fiber of the present invention;

FIG. 2 is an enlarged, partially schematic cross-sectionalrepresentation of another embodiment of a fiber of the presentinvention;

FIG. 3 is an enlarged plan view of a conventional tow yarn formed fromexpanded polytetrafluoroethylene (PTFE), with its filaments shown inpartially exploded orientation;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 3;

FIG. 5 is a partially schematic representation of a plan view ofconventional fiberglass yarn;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5;

FIG. 7 is a schematic representation of a reverse-air filter bag houseemploying filter bags constructed from fabric of the present invention;

FIG. 8 is a schematic representation of a shaker-type filter bag houseemploying filter bags constructed from fabric of the present invention;and

FIG. 9 is a schematic representation of the shaker bag house of FIG. 8,showing the filter bags being cleaned.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved textile which is a composite offiberglass filaments and polytetrafluoroethylene (PTFE) filaments.

As is illustrated in FIGS. 1 and 2, the fibers 10a, 10b of the presentinvention comprise filaments of fiberglass 12a, 12b and filaments ofPTFE 14a, 14b. The filaments 12, 14 are commingled together to form acohesive composite thread.

It should be understood that as the term "fiber" is applied herein, itis intended to encompass any strand of composite material made inaccordance with the present invention, regardless of application. Thisis intended to include, without limitation, instances where it isemployed as a single thread, as multiple threads braided or otherwisecombined together, as a non-woven fabric or membrane material, and as awoven fabric or membrane material.

As is explained in detail below, by varying the parameters used tocombine the fiberglass and PTFE filaments, a number of different formsof commingled fibers can be achieved. For example, FIG. 1 shows a fiber10a wherein clusters 16 of fiberglass filaments 12a tend to remainoriented together, with the PTFE filaments 14a essentially encapsulatingthe clusters 16. By contrast, the fiber 10b of FIG. 2 has a far morehomogenous mix of fiberglass filaments 12b and PTFE filaments 14b, withthe fiberglass filaments 12b oriented randomly or fairly evenlythroughout the PTFE.

It has been determined that by commingling the filaments of fiberglassand PTFE into a composite fiber of the present invention, a number ofunique and desirable properties can be imparted to the fiber. Unlikeprevious fiberglass fibers, the fibers of the present invention havedemonstrated exceptional flex and abrasion resistance. Additionally,these fibers have a lower coefficient of friction and natural lubricitywhen they rub against each other. They also have unique bulkycharacteristics, providing variability in the texture of the yarn,including its density and morphology.

Unlike previous fibers constructed from PTFE alone, these fibers haveproven to have better thermal stability, to experience less "creep"under standard operating conditions of heat and tension, and to survivetemperatures surges which would have shrunk or stretched PTFE.

To create the fiber 10 of the present invention, basically the filamentsof PTFE are spread apart into an open lattice and the filaments offiberglass are then inserted and enclosed within the lattice.Preferably, before or during the insertion process, the fiberglassfilaments are bulked, fragmented, and "texturized" into the PTFElattice.

The process of commingling the fibers is preferably accomplished throughuse of air-jet texturing apparatus. As is known, textured yarns arethose which have been given notably greater apparent volume than acorrelational yarn of similar fiber (filament) count and linear density.This is normally accomplished by distorting the shape and/or orientationof the fibers and then setting the fiber (usually through someapplication of heat) in the distorted orientation. Common procedures inthis regard are twisting, heating, crimping, and air jetting.

Air jet apparatus achieves bulk in a yarn by feeding the fibers overhigh pressure jets of air at a rate faster than they are drawn off. Thisdrives some of the fibers into random loops and provides a substantialamount of bulk. The amount of disarray of the fibers is a function ofmany parameters, including the amount of tension on the yarn, the speedof the yarn is driven through the apparatus, and the amount of airpressure applied.

Improvements in the basic air jet apparatus has allowed it to beemployed to combine two or more types of material into a yarn.Essentially, this process involves: (1) feeding a first material througha series of rollers and the air jet to open up its structure; and (2)simultaneously "overfeeding" a second material to cause it to be drawninto and interlaced within the first material. Again, by adjustingoperating parameters (e.g. the relative speed of yarn feed, air jetpressure, relative roller speed, temperature, etc.), a wide variety ofyarn textures and types can be produced. Among the products which arecreated with such an apparatus are single end yarns, multiple end yarns,core and effect yarns, novelty yarns (e.g. with different color mixes),and blended yarns (e.g. filament yarns with staple yarns blended in).One such apparatus is commercially available as a "Model ATM" fromHIRSCHBURGER-ELTEX U.S., INC., of Greer, S.C. 29650.

It has been determined that the present invention can be readilyconstructed by employing such air jet apparatus. By simultaneouslyfeeding PTFE and fiberglass through the air jet, the fiberglassfilaments become entrapped within the structure of the PTFE (or visaversa) and form a composite. By adjusting the relative rates of feed, acomposite can be readily created as is shown in FIGS. 1 and 2.

It has further been shown that some of the filaments of fiberglass tendto fragment when subjected to the air jet apparatus, thus forming acomposite fiber of continuous lengths of PTFE entrapping fragmentedfilaments of fiberglass. As has been noted, this is believed to producean even more flexible and abrasion resistant composite fiber than onewhere care is taken to retain the fiberglass in longer, more coherentstrands.

The processing of the fibers of the present invention may be betterunderstood by referring to the drawings of FIGS. 3-6. In the presentlypreferred application of the present invention, an expanded PTFE polymeris used. Such a product can be produced in a known manner, such as inaccordance with the teachings of U.S. Pat. No. 3,953,566 issued Apr. 27,1976, to Gore. Ideally, a slit film is created which is then passed overa series of ripper elements (e.g. a pin wheel) in a known manner to forma tow yarn 18. A representation of such a yarn is shown in FIG. 3.

As is shown in FIG. 3, when created in this manner, the tow yarn 18 canbe easily separated to reveal a lattice structure of long randomlyinterconnected filaments 20 of expanded PTFE. As is shown in FIG. 4, inun-exploded orientation, these filaments 20 are positioned within thePTFE tow yarn defining open spaces 22 between them.

On the other hand, as is shown in FIGS. 5 and 6, fiberglass filaments 24are generally arranged in essentially a parallel or spiral arrangement.When subjected to an air-jet texturing apparatus, some of thesefilaments normally become fragmented into shorter strands.

The fiberglass and PTFE can be mixed in various proportions to impartdifferent properties to the composite thread. Generally, the compositefiber should contain 10 to 75% by weight of PTFE and 25 to 90% by weightof fiberglass. These proportions are a function of the mechanicalproperties required for a given application, with relatively more PTFEincluded to provide greater flex fatigue and abrasion resistance. Formost applications, proportions of 25 to 50% by weight of PTFE and 50 to75% by weight fiberglass are preferred.

In addition, for some applications it may be desirable to include one ormore fillers to provide additional or enhanced properties for thecomposite fiber. Appropriate fillers include conductive fillers, such asgraphite, carbon black or metal oxide, to produce an electricallyconductive fiber; metal oxide or organic pigments to create colors foraesthetic or other reasons; and/or thermoplastic thermoset resins tocreate structural composites. Such fillers can be coated onto thecomposite fiber and/or incorporated into the PTFE fiber itself, such asthrough procedures similar to those disclosed in U.S. Pat. No. 4,985,298issued Jan. 15, 1991, to Mortimer, Jr.

The fibers created in accordance with the present invention can bereadily used to create fabrics with a wide assortment of uses. This canbe accomplished through any conventional procedures to create a woven,non-woven, braided or ribbon material.

Preferably, the fabric is woven on conventional weaving looms to producea woven fabric. The resulting fabric can be scoured with a surfactantand water solution or heat cleaned using a high temperature bake cycleto remove any residual sizing. Additional chemical finishes can then beapplied to the composite fabric to impart chemical resistance or otherdesired properties. One of the more promising applications of thepresent invention in this regard is to create woven fabrics for use asfilter media.

Shown in FIG. 7 is a representation of a reverse air filter bag house26. Such industrial filter units employ an inlet 28 for dirty fluegases, an outlet 30 for clean flue gases, and one or more filter bags32a, 32b, 32c, 32d in communication between the inlet and the outlet totrap particulate matter. In order to regenerate the filter bags 32 whenthey become internally encrusted with "dust cake" or "secondary cake," areverse air inlet 34 is provided for imparting a pressure onto theoutside of the filter bags 32.

As is shown with respect to bags 32c, 32d, when reverse air pressure isapplied, the fabric of the bags becomes distorted inward. Thecombination of reverse air flow and the inward distortion of the bagstends to dislodge particulate 36 into a hopper 38 where it can beremoved. Ribs or rings 40 are normally applied in this context to helpretain the overall shape of the bags 32 during reverse air flow.

Although fiberglass bags are commonly employed in this environment, thestresses caused by the reverse air flow weaken the filter bags at theareas of flex between the rings 40. Due to the brittle nature offiberglass, this results in relatively rapid bag deterioration whencompared to bags made from more flexible synthetic materials. Throughthe use of filter bags 32a, 32b, 32c, 32d constructed from a wovenfabric of the present invention, the life of such filter bags can bedramatically increased. It is believed that bag life can be increased onthe order of 2 to 10 times in this manner.

Further, the fiber flexibility imparted by the present invention permitsuse of fiberglass bags in far more demanding filter applications, suchas "shaker" type and pulse-jet type filter bags. As is shown in FIGS. 8and 9, a conventional shaker filter bag house 42 has an inlet 44 fordirty flue gases, one or more outlets 46 for clean flue gases, and oneor more filter bags 48a, 48b, 48c in communication between the inlet 44and the outlet 46 for trapping particulate matter 50. Similar inscrubbing operation to the reverse air bag house, during operation theinsides of the shaker bag filters 48 become encrusted with secondarycake as shown.

In order to regenerate the filter media, each of the filter bags 48 isattached to a rocker arm 52 actuated by rotating shaft 54 or similardevice which "shake" the bags to free particulate. The shaking procedureis illustrated in FIG. 9. When the bags are distorted in this manner,particulate 50 will fall into a hopper 56 where it can be removed.

It should be evident from the previous discussion of the fragile natureof fiberglass bags that they simply are unable to withstand the vigorousshaking of this environment without undergoing serious fatigue and earlyfailure. By contrast, bags 48a, 48b, 48c of the present invention caneasily withstand such treatment without failure or loss ofeffectiveness.

Additionally, the fabric of the present invention lends itself to manyfurther improvements in filter environments. For example, a compositefabric of the present invention can be combined with other filtercoatings (e.g. an expanded PTFE filter membrane) to provide improvedfiltration characteristics. The strong yet flexible nature of the fiberof the present invention is believed to be able to create a superiorfilter in this regard.

Further, by employing conductive filled PTFE tow yarns, it is possibleto dissipate static charges created by tribo-electrically dissimilarmaterials rubbing against each other (e.g. dust rubbing against thefilter bag surface). Furthermore, a variety of other improved filterproperties are believed possible through use of the present invention.For example, flexible release belts can be improved by weaving a textileusing the fiber of the invention. By coating such belts with TEFLON Bdispersion or similar material, an improved release belt surface can beprovided for cooking or sealing operations.

The fiber and fabric of the present invention has numerous otherpossible applications. For instance, it can be employed as an improvedsewing thread, belting, architectural fabric, in printed circuit boards,as a structural composite, in roofing material, as insulation material,etc.

Without intending to limit the scope of the present invention, thenature and operation of the present invention can be better understoodwhen considered in light of the following example.

EXAMPLE

A 1200 denier expanded PTFE monofilament slit film fiber was towed in acorrelational manner using a pinwheel. A fiberglass yarn with thedescription ECDE 75 2/0, commercially available from Owens CorningFiberglass or PPG Industries, was employed to combine with the PTFE.

The two yarns in equal proportions (50:50) were twisted together at 2.8turns per inch in the S direction. The twisted yarn was then run througha HIRSCHBURGER-ELTEX Air Texturing machine, Model ATM, to intimatelyblend or commingle the fibers. Once commingled, it was impossible toseparate the component yarns without destroying them. The yarn now had a"fuzzy" appearance.

The following table is a physical property comparison of the commingledyarn to a conventional fiberglass yarn.

    ______________________________________                                        Fill Yarn Type Conventional Yarn                                                                          Composite Yarn                                    ______________________________________                                        Description    ECDE 75 1/4 Tex                                                                            ECDE 75 2/0 +                                                                 PTFE                                              Denier         2400         2400                                              Break Strength 4.84         14.7                                              (lbs.)                                                                        M.I.T. Flex Cycles                                                                           8            1408                                              to Failure                                                                    Yarn Abrasion Cycles                                                                         0            188                                               to Failure                                                                    ______________________________________                                    

The "M.I.T. Flex Cycles" test comprises an apparatus which double foldsthe fiber over a straightedge in either direction over a 270° rangeuntil failure occurs. The "Yarn Abrasion Cycles" test comprises fixingtwo similar yarns in brackets and then rubbing the yarn on itself untilfailure occurs. A cycle is one complete stroke which includes forwardand return movements. The conventional fiberglass yarn tested wasincapable of completing one cycle.

The above fiber was woven on a conventional weaving loom to produce a 22oz./yd² greige fabric. The greige fabric had the following properties:thread count of 48×40 threads per inch; thickness of 0.034 inches; andbreak strength (W×F) of 570×323 lbs/inch grab.

The greige fabric was scoured with a hot water and surfactant solutionto remove starches and weaving aids. It was then coated with TEFLON Bdispersion and cured in a convection oven. The fabric was baked at 500°F. overnight to heat set the fabric and bake off any residualsurfactants from the coating process.

For use of this fiber as a filtration fiber, the demands of filtrationapplication require improved flex fatigue resistance in the fillingdirection only. As such, the fibers of the present invention need onlybe applied in that direction.

The fabric so constructed was then tested for physical properties andfound to possess the following as compared to conventional fiberglassfiltration media:

    ______________________________________                                                    CONVENTIONAL                                                      CHARACTERISTIC                                                                            FABRIC       COMPOSITE FABRIC                                     ______________________________________                                        Warp Yarn   ECDE 75 1/2  ECDE 75 1/2                                          Fill Yarn   ECDE 75 1/4 TEX                                                                            2 ENDS ECDE 75 1/0                                                            1 END 1200 DENIER                                                             PTFE                                                 Weight (Oz/Yd.sup.2)                                                                      24.0         23.7                                                 Thickness (inches)                                                                        0.034        0.040                                                Permeability                                                                              60           46                                                   Thread Count                                                                              47 × 40                                                                              48 × 40                                        Break Strength                                                                            500 × 400                                                                            510 × 445                                      M.I.T. Flex 50,000 × 7000                                                                          50,000 × 1,000,000                           Endurance                                                                     Water Drop  OK           OK                                                   Resistance                                                                    ______________________________________                                    

The composite fabric was tested for filtration efficiency and found tohave significantly better capture efficiency than conventionalfiberglass fabrics. The composite fabric was also successfully laminatedwith a microporous PTFE membrane. This fabric is expected to providesuperior performance during field testing.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

The invention claimed is:
 1. A method of producing an improved compositefabric, which comprises:providing filaments of fiberglass; providingfilaments of polytetrafluoroethylene (PTFE); commingling the filamentsof fiberglass and PTFE by air-jet texturing the filaments so as toseparate the filaments of PTFE and to fragment filaments of fiberglassto form a strand of composite fiber, wherein fragments of fiberglassfilaments are interspersed within the filaments of PTFE; and weavingmultiple composite fibers into a fabric.
 2. The method of claim I whichfurther comprises providing PTFE filaments in the form of a tow yarnformed from expanded PTFE.
 3. The method of claim 1 which furthercomprises providing fiberglass filaments in a relative proportion of 10to 75% by weight of the fiber.
 4. The method of claim 3 which furthercomprises providing PTFE filaments in the form of a tow yarn comprising25 to 90% by weight of the fiber.
 5. The method of claim 1 which furthercomprises weaving multiple composite fibers into a fabric.
 6. The methodof claim 5 which further comprises attaching a membrane of expanded PTFEto the fabric.
 7. The method of claim 6 which further comprises formingthe fabric and attached membrane of expanded PTFE into a filter.